Unwanted and/or uncoordinated generation of nerve impulses may be a disabling factor in some medical conditions. For example, uncoordinated motor signals may produce spasticity in stroke, cerebral palsy, multiple sclerosis, and other conditions. The uncoordinated signals may result in the inability to make desired functional movements. Involuntary motor signals in conditions including tics, choreas, and so on, may produce unwanted movements. Additionally, unwanted sensory signals can cause pain. Conventional approaches have attempted to intercept unwanted or uncoordinated nerve impulses along the nerves on which they travel to attempt to reduce and/or eliminate the disabling condition.
Conventional approaches associated with treating these conditions have produced unsatisfactory results. For example, drug treatments may have produced unwanted side-effects, may have acted globally on the body rather than specifically on a specific nerve, and may have been neither quick acting nor quickly reversible. While chemical treatments (e.g., Botox, phenol blocks), may be applied more specifically, they may have been destructive to the nerve, may have required reapplication, and may not have been quickly reversible. Other conventional treatments for pain (e.g., transcutaneous electrical nerve stimulation (TENS), implantable pain stimulators) have also produced sub-optimal results.
Both alternating current (AC) and direct current (DC) nerve stimulation are known in the art. The inhibitory effect of high-frequency alternating current (HFAC) on nerves has been reported since the early 1900's. Additionally, DC electrical nerve stimulation has been illustrated to produce a nearly complete block of nerve activity. However, conventional DC stimulation has damaged both body tissues and/or electrodes when delivered over prolonged periods of time. Thus conventional DC stimulation has been unsuitable for certain applications. The damage caused by a DC nerve block is due, at least in part, to unbalanced charge applied to the nerve. HFAC, which delivers a zero net charge to the tissue is likely to be safer as a method for nerve block. However, when HFAC is delivered to a nerve, it causes a burst of activity in the nerve that is undesirable and likely to be painful. The burst of activity produced by HFAC is referred to as the onset activity.